It is known to mount an integrated circuit die to a printed circuit board by solder bump interconnections. The integrated circuit die is spaced apart from the printed circuit board by a gap. The solder bump interconnections extend across the gap and connect bond pads on the integrated circuit die to bond pads on printed circuit boards to attach the die and conduct electrical signals to and from the die for processing. Because of differences between the coefficients of thermal expansion of the die and the board, stresses are created when the assembly is subjected to thermal cycling during use. These stresses tend to fatigue the solder bump interconnections and can lead to failure of the assembly. In order to strengthen the solder joints without affecting the electrical connection, the gap is filled with a polymeric encapsulant, typically a filled polymer.
The encapsulant is typically applied after the solder bumps are reflowed to bond the integrated circuit die to the printed circuit board. A polymeric precursor is dispensed onto the board adjacent to the die and is drawn into the gap by capillary action. The precursor is then heated and cured to form the encapsulant. This curing can also create stresses that can be detrimental to the die and the interconnections.
Therefore, a need exists to reduce the detrimental effects of thermally induced stresses upon a microelectronic assembly wherein the integrated circuit die is attached to the printed circuit board by solder bump interconnections and encapsulated within a polymeric material, and to thereby extend the useful life of the assembly.